This research was supported by the Intramural Research Program of the NIH, NCI, and CCR. All authors received Intramural Research Grant from NIH, NCI, and CCR.

1. Prepare Reagents
<ol>
	<li>1x PBS
	<ol>
		<li>Dilute 100 mL 10x PBS with 900 mL dH2O and adjust the pH to 7.4 using a pH meter. Store at room temperature.</li>
	</ol>
	</li>
	<li>Lysis solution (LS)
	<ol>
		<li>Prepare 2.5 M NaCl, 100 mM disodium EDTA, 10 mM Tris base, and 200 mM NaOH in 900 mL dH2O; it commonly takes about 20 min to allow the mixture to fully dissolve. Adjust the pH to 10 using a pH meter. Add 1% sodium lauryl sarcosinate and 1% Triton X-100, and adjust the final volume to 1,000 mL. Cool to 4 &#176;C for at least 30 min before use.</li>
	</ol>
	</li>
	<li>Alkaline electrophoresis solution (AES), pH &#62;13
	<ol>
		<li>Prepare 200 mM NaOH and 1 mM disodium EDTA in 800 mL dH2O. Adjust the pH and make sure that it is pH &#62;13. Adjust the final volume to 1,000 mL. Make fresh before use and cool to 4 &#176;C for at least 30 min before use.</li>
	</ol>
	</li>
	<li>Neutral electrophoresis sollution (NES)
	<ol>
		<li>Prepare 1,000 mL neutral electrophoresis buffer by mixing 100 mM Tris base and 300 mM sodium acetate to 1,000 mL dH2O. Adjust the pH to 9.0 with glacial acetic acid. Cool to 4 &#176;C for at least 30 min before use.</li>
	</ol>
	</li>
	<li>DNA precipitation solution (DPS)
	<ol>
		<li>Preparation of 10 mL 7.5 M ammonium acetate stock. For 50 mL of DNA precipitation solution, mix 6.7 mL 7.5 M ammonium acetate with 43.3 mL 95% ethanol. Store at room temperature.</li>
	</ol>
	</li>
	<li>Staining solution
	<ol>
		<li>Add 1 &#181;L 10,000x&#160;green fluorescent nucleic acid stain (e.g., SYBR Green) in 30 mL Tris-EDTA buffer (10 mM Tris-HCl, 1 mM disodium EDTA, pH 7.4) and store at 4 &#176;C. Protect from light.</li>
	</ol>
	</li>
	<li>1% low melting agarose
	<ol>
		<li>Melt 1% low melting point agarose (1 g in 100 mL dH2O) in a microwave. Swirl the agarose every 15-20 s to make sure that the agarose is completely molten. Place the agarose in 37 &#176;C water bath for at least 20 min before use.</li>
	</ol>
	</li>
	<li>Pre-warm pipette tips
	<ol>
		<li>Cut off the narrow ends of P200 pipette tips by 3 mm and warm at 37 &#176;C before pipetting agarose.</li>
	</ol>
	</li>
</ol>
2. Prepare Comet Slides
<ol>
	<li>Slide coating
	<ol>
		<li>Melt 1% agarose (1 g in 100 mL dH2O) in a microwave for 2 - 3 min or until the agarose is completely molten. Dip the glass microscope slides into the agarose and wipe one side of the slide using a lint-free wipe.</li>
		<li>Lay the slides on a flat surface to air-dry or heat at 50 &#176;C for faster drying; a transparent agarose film should be formed after drying. Place the coated slides in 37 &#176;C before use.</li>
	</ol>
	</li>
	<li>Preparation of single cell suspensions
	<ol>
		<li>Culture and treat the glioma cell
		<ol>
			<li>Culture the U251 MG cells in DMEM-Ham F-12 medium supplemented with 10% FBS, 100 U/mL penicillin, and 10 &#181;g/mL streptomycin at 37 &#176;C with 5% CO2.</li>
			<li>Digest the cells using 1 mL trypsin for 3 min, and neutralize trypsin using DMEM-Ham F-12 medium with FBS. Collect in 15 mL tube, spin at 300 x g for 4 min, aspirate the medium, and suspend cells at 2 x 105 cell/mL in 1x PBS. 
			NOTE: The cell sample should be prepared immediately before starting the assay and all samples should be handled in a dark or dimmed environment to prevent DNA damage from light.</li>
			<li>Combine the cell suspension with 1% molten low melting point agarose (at 37 &#176;C) at a ratio 1:10 (v/v), mix gently by pipetting up and down, and immediately pipette 30 &#181;L onto a slide. Use the side of the the pipette tip to spread the agarose/cell mixture to ensure the formation of a thin layer.</li>
			<li>Place the slide flat at 4 &#176;C in the dark for 10 min. Increasing the gelling time to 30 min improves adherence of samples in high humidity environments.</li>
			<li>Immerse the slide in 4 &#176;C LS in the dark for 1 h to overnight.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
3. Single Cell Electrophoresis
<ol>
	<li>Proceed to alkaline (step 3.2) or neutral (step 3.3) comet assay</li>
	<li>For alkaline comet assay
	<ol>
		<li>Gently remove slides from the LS, drain excess buffer, and gently immerse in AES for 1 h at 4 &#176;C to allow DNA unwinding. Keep the slides in the dark.</li>
		<li>Add pre-chilled AES in the electrophoresis slide tray, do not exceed 0.5 cm above the slides (this depends on the size of the electrophoresis units), place the slides inside and cover with a cap. Set the power supply voltage to 1 V/cm (the length between electrodes) and run for 30 min at 4 &#176;C.</li>
		<li>Drain excess electrophoresis solution from slide. Gently immerse slides twice in dH2O for 5 min each at room temperature.</li>
		<li>Gently immerse slides in 70% ethanol for 5 min at room temperature. Proceed to step 4.</li>
	</ol>
	</li>
	<li>For neutral comet assay
	<ol>
		<li>Gently remove the slides from the LS, drain excess buffer, and gently immerse in NES for 30 min at 4 &#176;C. Keep the slide in the dark.</li>
		<li>Add pre-chilled neutral electrophoresis buffer in the electrophoresis slide tray, do not exceed 0.5 cm above slides (this depends on the size of the electrophoresis units), place the slides inside and cover with a cap. Set the power supply voltage to 1 V/cm (the length between electrodes) and run for 45 min at 4 &#176;C.</li>
		<li>Drain excess buffer from the slides. Gently immerse slides&#160;in DPS for 30 min at room temperature.</li>
		<li>Gently immerse the slides in 70% ethanol for 30 min at room temperature. Proceed to step 4.</li>
	</ol>
	</li>
</ol>
4. Stain Comet Slides
<ol>
	<li>Dry slides at 37 &#176;C for 10 - 15 min in the dark.</li>
	<li>Place 50 - 100 &#181;L green fluorescent nucleic acid staining solution onto each dried agarose and stain for 15 min at room temperature in the dark.</li>
	<li>Rinse the slides briefly in dH2O and dry completely at 37 &#176;C in the dark. Proceed to image acquisition and analysis.</li>
</ol>
5. Image Acquisition and Analysis
NOTE: The visualization and quantification of DNA breaks are based on epifluorescence microscopy and the comet assay software (see Table of Materials)12.
<ol>
	<li>Place the slides on the microscope with a slide holder. Ensure the agarose gel is facing to the objective lens. Randomly capture images from the stained comet slides using a fluorescence microscope with a 10x objective lens. Avoid the edges and the areas around any air bubbles.</li>
	<li>Ensure each comet tail is horizontally distributed. Comet heads should originate from the left and the tail from the right.</li>
	<li>Save each picture in a binary TIF format with bright DNA stain and dark background. Load images to the software using the &#34;Select files to analyze&#34; button, which is located on the left of the toolbar. An image view window should appear (Figure 1).</li>
	<li>Draw a measurement frame on the screen and adjust its size in accordance with the comet of the cell. Click the &#34;Adjust&#34; button to set up the threshold of the head, comet, and tail according to the image, then click the &#34;Start measurements&#34; button (Figure 1).</li>
	<li>Select a cell using the frame and activate the measurement by clicking with the mouse on the &#34;Assay the comet&#34; button; an intensity image shows up on the &#34;profiles&#34; window with the selected measurement parameters. The results can be saved by clicking the &#34;store result&#34; button (Figure 1). 
	NOTE: The software calculates the parameters including the length of the comet tail, the percentage of the tailed DNA, the tail moment (TM) and the Olive tail moment (OTM). The tail moments are calculated by the formulas as follow: 
	<img alt="Equation 1" src="/files/ftp_upload/56450/56450eq1.jpg" /> 
	<img alt="Equation 2" src="/files/ftp_upload/56450/56450eq2.jpg" /></li>
	<li>Analyze at least 50 cells per treatment.</li>
</ol>

<ol>
	<li>Ostling, O., Johanson, K. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microelectrophoretic+study+of+radiation-induced+DNA+damages+in+individual+mammalian+cells.">Microelectrophoretic study of radiation-induced DNA damages in individual mammalian cells.</a> Biochem Biophys Res Commun. 123 (1), 291-298 (1984).</li><li>Singh, N. P., McCoy, M. T., Tice, R. R., Schneider, E. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+simple+technique+for+quantitation+of+low+levels+of+DNA+damage+in+individual+cells.">A simple technique for quantitation of low levels of DNA damage in individual cells.</a> Exp Cell Res. 175 (1), 184-191 (1988).</li><li>Tice, R. R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Single+cell+gel/comet+assay:+guidelines+for+in+vitro+and+in+vivo+genetic+toxicology+testing.">Single cell gel/comet assay: guidelines for in vitro and in vivo genetic toxicology testing.</a> Environ Mol Mutagen. 35 (3), 206-221 (2000).</li><li>Shah, A. J., Lakkad, B. C., Rao, M. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Genotoxicity+in+lead+treated+human+lymphocytes+evaluated+by+micronucleus+and+comet+assays.">Genotoxicity in lead treated human lymphocytes evaluated by micronucleus and comet assays.</a> Indian J Exp Biol. 54 (8), 502-508 (2016).</li><li>Azqueta, A., Collins, A. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+essential+comet+assay:+a+comprehensive+guide+to+measuring+DNA+damage+and+repair.">The essential comet assay: a comprehensive guide to measuring DNA damage and repair.</a> Arch Toxicol. 87 (6), 949-968 (2013).</li><li>Goldstein, M., Kastan, M. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+DNA+damage+response:+implications+for+tumor+responses+to+radiation+and+chemotherapy.">The DNA damage response: implications for tumor responses to radiation and chemotherapy.</a> Annu Rev Med. 66, 129-143 (2015).</li><li>Gavande, N. S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=DNA+repair+targeted+therapy:+The+past+or+future+of+cancer+treatment?.">DNA repair targeted therapy: The past or future of cancer treatment?.</a> Pharmacol Ther. 160, 65-83 (2016).</li><li>Torgovnick, A., Schumacher, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=DNA+repair+mechanisms+in+cancer+development+and+therapy.">DNA repair mechanisms in cancer development and therapy.</a> Front Genet. 6, 157 (2015).</li><li>Weston, V. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+PARP+inhibitor+olaparib+induces+significant+killing+of+ATM-deficient+lymphoid+tumor+cells+in+vitro+and+in+vivo.">The PARP inhibitor olaparib induces significant killing of ATM-deficient lymphoid tumor cells in vitro and in vivo.</a> Blood. 116 (22), 4578-4587 (2010).</li><li>Brown, J. S., O'Carrigan, B., Jackson, S. P., Yap, T. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Targeting+DNA+Repair+in+Cancer:+Beyond+PARP+Inhibitors.">Targeting DNA Repair in Cancer: Beyond PARP Inhibitors.</a> Cancer Discov. 7 (1), 20-37 (2017).</li><li>Lu, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chemosensitivity+of+IDH1-Mutated+Gliomas+Due+to+an+Impairment+in+PARP1-Mediated+DNA+Repair.">Chemosensitivity of IDH1-Mutated Gliomas Due to an Impairment in PARP1-Mediated DNA Repair.</a> Cancer Res. 77 (7), 1709-1718 (2017).</li><li>Konca, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+cross-platform+public+domain+PC+image-analysis+program+for+the+comet+assay.">A cross-platform public domain PC image-analysis program for the comet assay.</a> Mutat Res. 534 (1-2), 15-20 (2003).</li><li>Valverde, M., Rojas, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Environmental+and+occupational+biomonitoring+using+the+Comet+assay.">Environmental and occupational biomonitoring using the Comet assay.</a> Mutat Res. 681 (1), 93-109 (2009).</li><li>Collins, A. 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The authors have nothing to disclose.

The comet assay is an efficient tool to measure single and double-strand DNA breaks at the cellular level. The assay has been widely applied as a "golden standard" in studies regarding genotoxicity and biomonitoring13, ranging from base lesions, DNA crosslinks, drug development, and alkali sensitive sites. In the present study, we showed two distinct step-by-step protocols for alkaline and neutral comet assays, respectively. Combining single cell electrophoresis, fluorescent microscopy, and image ...

The comet assay was first developed by Ostling and Johanson in 1984 by demonstrating the migration of DNA fragments from nuclei under a neutral condition1. The technique was later developed by Singh et al., showing that an alkaline condition substantially increased the specificity and reproducibility of the assay2. Since then, the neutral comet assay is mostly used to detect double-stranded DNA breaks, whereas the alkaline comet assay is more sensitive for smaller amounts of DNA damage, including single and double strand DNA breaks, alkali-labile sites, DNA-DNA or DNA-protein cross-linking, and DNA single-strand breaks associated with incomplete excision repair sites3,4. Both assays allow visualization of fragmented DNA, and provide a straightforward way to quantitatively evaluate DNA damage. The comet assay is regarded as a sensitive method for in vitro and in vivo genetic toxicological studies and is applicable to different research areas, such as early drug-candidate selection, environmental monitoring, human biomonitoring, and fundamental research in DNA damage and repair5. 
The principle of the assay is that under an electric field, fragmented DNA migrates out of the nucleoid body (also known as the "comet head") and forms a DNA stain in the agarose gel (also known as the "comet tail"). With nucleotide staining, the extent of DNA damage can be quantified by analyzing "comets" formed by this single cell electrophoresis. Calculation of the tail moment can further help to compare DNA damage among different experimental groups. Compared with traditional methods of DNA damage detection, the comet assay is direct, sensitive, inexpensive, and relatively simple.
Radiotherapy and chemotherapies are common strategies for cancer treatment by generating single strand and double strand DNA breaks in chromosomes6. The recent advancement in DNA repair inhibitors allows a more effective genotoxic effect by combination chemotherapy, and therefore, potentially reduces systemic side effects such as anemia, infection, and bone marrow suppression7,8. In this study, we showed the investigation of a poly (ADP-ribose) polymerase (PARP) inhibitor, olaparib (Ola)9. PARP is an abundant nuclear protein and is responsible for DNA base excision repair by forming a poly (ADP-ribose) polymer10. Temozolomide (TMZ) is an orally available alkylating agent and has been widely used for glioma patient treatment. By using the comet assay to quantify DNA damage, we demonstrate that combining olaparib with temozolomide profoundly enhances the DNA damage in glioma cells, which suggests olaparib/temozolomide combination therapy is an effective strategy to treat glioma, as compared with temozolomide alone11.

<table border="0" cellpadding="0" cellspacing="0" width="603">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Reagents</td>
			<td style="width:101px;"></td>
			<td style="width:119px;"></td>
			<td style="width:167px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:216px;">10x PBS(Ca++, Mg++ free)</td>
			<td style="width:101px;">TEKnova</td>
			<td style="width:119px;">P0196</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">NaCl</td>
			<td style="width:101px;">Sigma</td>
			<td style="width:119px;">S5886</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">EDTA</td>
			<td style="width:101px;">TEKnova</td>
			<td style="width:119px;">E0308</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Trizma base</td>
			<td style="width:101px;">Sigma</td>
			<td style="width:119px;">T1503</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">NaOH</td>
			<td style="width:101px;">Sigma</td>
			<td style="width:119px;">72068</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sodium lauryl sarcosinate</td>
			<td style="width:101px;">Sigma</td>
			<td style="width:119px;">L7414</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Triton X-100</td>
			<td style="width:101px;">Sigma</td>
			<td style="width:119px;">93443</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Sodium acetate</td>
			<td style="width:101px;">Sigma</td>
			<td style="width:119px;">32318</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Glacial acetic acid</td>
			<td style="width:101px;">Sigma</td>
			<td style="width:119px;">695092</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Ammonium acetate</td>
			<td style="width:101px;">Sigma</td>
			<td style="width:119px;">A1542</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">SYBR Green</td>
			<td style="width:101px;">Invitrogen</td>
			<td style="width:119px;">S33102</td>
			<td></td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:216px;">Low melting point agarose</td>
			<td style="width:101px;">Invitrogen</td>
			<td style="width:119px;">16520</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Agarose</td>
			<td style="width:101px;">Invitrogen</td>
			<td style="width:119px;">16500</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">95% ethanol</td>
			<td style="width:101px;">WARNER-GRAHAM</td>
			<td style="width:119px;">#64-17-5</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Trypsin</td>
			<td style="width:101px;">GIBICO</td>
			<td style="width:119px;">25300-054</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Name</td>
			<td style="width:101px;">Company</td>
			<td style="width:119px;">Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Consumables</td>
			<td style="width:101px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:216px;">Glass tissue slides</td>
			<td style="width:101px;">ELECTRON MICROSCOPY SCIENCES</td>
			<td style="width:119px;">63422-11</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Kimwipes</td>
			<td style="width:101px;">KIMberly-Clark</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">1.5 mL Microcentrifuge Tubes</td>
			<td style="width:101px;">DENVILLE</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Pipette Tips</td>
			<td style="width:101px;">SHARP</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Name</td>
			<td style="width:101px;">Company</td>
			<td style="width:119px;">Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Equipments</td>
			<td style="width:101px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Microwave</td>
			<td style="width:101px;">Avanti</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Waterbath</td>
			<td style="width:101px;">PRECISION</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:216px;">Horizontal electrophoresis chamber</td>
			<td style="width:101px;">TREVIGEN</td>
			<td style="width:119px;">Cometassay ES II</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Power supply</td>
			<td style="width:101px;">Bio-Rad</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Incubator</td>
			<td style="width:101px;">Quincy Lab</td>
			<td style="width:119px;">Model 12-140E</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Fluorescent microscope</td>
			<td style="width:101px;">Zeiss</td>
			<td style="width:119px;">LSM700</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:216px;">Micropipettor</td>
			<td style="width:101px;">Eppendorf</td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
	</tbody>
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evaluating in vitro dna damage using comet assay

DNA damage is a common phenomenon for each cell during its lifespan, and is defined as an alteration of the chemical structure of genomic DNA. Cancer therapies, such as radio- and chemotherapy, introduce enormous amount of additional DNA damage, leading to cell cycle arrest and apoptosis to limit cancer progression. Quantitative assessment of DNA damage during experimental cancer therapy is a key step to justify the effectiveness of a genotoxic agent. In this study, we focus on a single cell electrophoresis assay, also known as the comet assay, which can quantify single and double-strand DNA breaks in vitro. The comet assay is a DNA damage quantification method that is efficient and easy to perform, and has low time/budget demands and high reproducibility. Here, we highlight the utility of the comet assay for a preclinical study by evaluating the genotoxic effect of olaparib/temozolomide combination therapy to U251 glioma cells.

The present protocol describes a step-by-step workflow for the comet assay execution and data analysis (Figure 1). Results from the alkaline and neutral comet assays showed that the comet tail of doxorubicin-treated U251 cells (1 &#181;M, 20 h) was longer and had higher DNA intensity, suggesting a substantial accumulation of fragmented DNA due to chemotherapy (Figure 2).
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Evaluating In Vitro DNA Damage Using Comet Assay

The comet assay is an efficient method to detect DNA damage including single and double-stranded DNA breaks. We describe alkaline and neutral comet assays to measure DNA damage in cancer cells to evaluate the therapeutic effect of chemotherapy.

Evaluating In Vitro DNA Damage Using Comet Assay

DNA damage is a common phenomenon for each cell during its lifespan, and is defined as an alteration of the chemical structure of genomic DNA. Cancer ...

evaluating-in-vitro-dna-damage-using-comet-assay

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Cancer Research

36.9K Views.  National Cancer Institute. The comet assay is an efficient method to detect DNA damage including single and double-stranded DNA breaks. We describe alkaline and neutral comet assays to measure DNA damage in cancer cells to evaluate the therapeutic effect of chemotherapy.

Video: Evaluating In Vitro DNA Damage Using Comet Assay

Moving Upwards: A Simple and Flexible In Vitro Three-dimensional Invasion Assay Protocol

Although 3D invasion assays have been developed, the challenge remains to study cells without affecting the integrity of their microenvironment. Traditional 3D assays such as the Boyden Chamber require that cells are displaced from the original culture location and moved to a new environment. Not only does this disrupt the cellular processes that are intrinsic to the microenvironment, but it often results in a loss of cells. These problems are especially challenging when dealing with cells that are either rare, or extremely sensitive to their microenvironment. Here, we describe the development of a 3D invasion assay that avoids both concerns. In this assay, cells are plated within a small well and an ECM matrix containing a chemoattractant is laid atop the cells. This requires no cell displacement, and allows the cells to invade upwards into the matrix. In this assay, cell invasion as well as cell morphology can be assessed within the collagen gel. Using this assay, we characterize the invasive capacity of rare and sensitive cells; the hybrid cells resulting from fusion between breast cancer cells MCF7 and mesenchymal/multipotent stem/stroma cells (MSCs).

Moving Upwards: A Simple and Flexible In Vitro Three-dimensional Invasion Assay Protocol

This protocol describes an inverted vertical invasion assay that could be used to quantify the migration and invasion capabilities of cells in a three-dimensional setting while preserving the cell microenvironment. This assay is suitable for rare and/or sensitive cells.

Although 3D invasion assays have been developed, the challenge remains to study cells without affecting the integrity of their microenvironment. ...

A Comprehensive Procedure to Evaluate the In Vitro Performance of the Putative Hemangioblastoma Neovascularization Using the Spheroid Sprouting Assay

The inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene plays a crucial role in the development of hemangioblastomas (HBs) within the human central nervous system (CNS). However, both the cytological origin and the evolutionary process of HBs (including neovascularization) remain controversial, and anti-angiogenesis for VHL-HBs, based on classic HB angiogenesis, have produced disappointing results in clinical trials. One major obstacle to the successful clinical translation of anti-vascular treatment is the lack of a thorough understanding of neovascularization in this vascular tumor. In this article, we present a comprehensive procedure to evaluate in vitro whether classic tumor angiogenesis exists in HBs, as well as its role in HBs. With this procedure, researchers can accurately understand the complexity of HB neovascularization and identify the function of this common form of angiogenesis in HBs. These protocols can be used to evaluate the most promising anti-vascular therapy for tumors, which has high translational potential either for tumors treatment or for aiding in the optimization of the anti-angiogenic treatment for HBs in future translations. The results highlight the complexity of HB neovascularization and suggest that this common form angiogenesis is only a complementary mechanism in HB neovascularization.

A Comprehensive Procedure to Evaluate the In Vitro Performance of the Putative Hemangioblastoma Neovascularization Using the Spheroid Sprouting Assay

This paper presents a comprehensive procedure to evaluate in vitro whether classic tumor angiogenesis exists in hemangioblastomas (HBs) and its role in HBs. The results highlight the complexity of HB-neovascularization and suggest that this common form of angiogenesis is only a complementary mechanism in the HB-neovascularization.

The inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene plays a crucial role in the development of hemangioblastomas (HBs) within the human ...

Evaluating the Role of Mitochondrial Function in Cancer-related Fatigue

Fatigue is a common and debilitating condition that affects most cancer patients. To date, fatigue remains poorly characterized with no diagnostic test to objectively measure the severity of this condition. Here we describe an optimized method for assessing mitochondrial function of PBMCs collected from fatigued cancer patients. Using a compact extracellular flux system and sequential injection of respiratory inhibitors, we examined PBMC mitochondrial functional status by measuring basal mitochondrial respiration, spare respiratory capacity, and energy phenotype, which describes the preferred energy pathway to respond to stress. Fresh PBMCs are readily available in the clinical setting using standard phlebotomy. The entire assay described in this protocol can be completed in less than 4 hours without the involvement of complex biochemical techniques. Additionally, we describe a normalization method that is necessary for obtaining reproducible data. The simple procedure and normalization methods presented allow for repeated sample collection from the same patient and generation of reproducible data that can be compared between time points to evaluate potential treatment effects.

Our goal was to develop a practical protocol to evaluate mitochondrial dysfunction associated with fatigue in cancer patients. This innovative protocol is optimized for clinical use involving only standard phlebotomy and basic laboratory procedures.

Fatigue is a common and debilitating condition that affects most cancer patients. To date, fatigue remains poorly characterized with no diagnostic test to ...

Real Time Detection of In Vitro Tumor Cell Apoptosis Induced by CD8+ T Cells to Study Immune Suppressive Functions of Tumor-infiltrating Myeloid Cells

Potentiation of the tumor-killing ability of CD8+ T cells in tumors, along with their efficient tumor infiltration, is a key element of successful immunotherapies. Several studies have indicated that tumor infiltrating myeloid cells (e.g., myeloid-derived suppressor cells (MDSCs) and tumor-associated macrophages (TAMs)) suppress cytotoxicity of CD8+ T cells in the tumor microenvironment, and that targeting these regulatory myeloid cells can improve immunotherapies. Here, we present an in vitro assay system to evaluate immune suppressive effects of monocytic-MDSCs and TAMs on the tumor-killing ability of CD8+ T cells. To this end, we first cultured na&#239;ve splenic CD8+ T cells with anti-CD3/CD28 activating antibodies in the presence or absence of suppressor cells, and then co-cultured the pre-activated T cells with target cancer cells in the presence of a fluorogenic caspase-3 substrate. Fluorescence from the substrate in cancer cells was detected by real-time fluorescence microscopy as an indicator of T-cell induced tumor cell apoptosis. In this assay, we can successfully detect the increase of tumor cell apoptosis by CD8+ T cells and its suppression by pre-culture with TAMs or MDSCs. This functional assay is useful for investigating CD8+ T cell suppression mechanisms by regulatory myeloid cells and identifying druggable targets to overcome it via high throughput screening.

Real Time Detection of In Vitro Tumor Cell Apoptosis Induced by CD8+ T Cells to Study Immune Suppressive Functions of Tumor-infiltrating Myeloid Cells

We describe here a protocol to investigate cytotoxicity of pre-activated CD8+ T cells against cancer cells by detecting apoptotic cancer cells via real-time microscopy. This protocol can investigate mechanisms behind myeloid cell-induced T cell suppression and evaluate compounds aimed at replenishing T cells via blockade of immune suppressive myeloid cells.

Potentiation of the tumor-killing ability of CD8+ T cells in tumors, along with their efficient tumor infiltration, is a key element of successful ...

In Vitro Assay to Study Tumor-macrophage Interaction

Tumor associated macrophages (TAMs) account for a large percentage of cells in the tumor mass for different types of cancers. Glioblastoma (GBM), a malignant brain tumor with no cure, has up to a half the tumor mass TAMs. TAMs can be pro-tumoral or anti-tumoral, depending on the activation of specific genes in the cells. Genetic mutations in the tumors, through regulating cytokine expression, can affect recruitment of TAMs to the tumor microenvironment. Here, we describe a quantitative cell-based assay to assess macrophage recruitment by the conditioned medium from the tumor cells. This assay uses the human macrophage cell line MV-4-11 to study macrophage attraction by the conditioned medium from glioblastoma, allowing for high reproducibility and low variability. Data generated with this assay can contribute to a better understanding of the interaction between the tumor and the tumor microenvironment. Similar assay can be used to assess interaction between the tumor cells and other immune cells, including T cells and natural killer (NK) cells.

This article represents a useful in vitro assay to evaluate the capability of conditioned medium from tumor cells to attract macrophages.

Tumor associated macrophages (TAMs) account for a large percentage of cells in the tumor mass for different types of cancers. Glioblastoma (GBM), a ...

Evaluating the Differentiation Capacity of Mouse Prostate Epithelial Cells Using Organoid Culture

The prostate epithelium is comprised predominantly of basal and luminal cells. In vivo lineage tracing has been utilized to define the differentiation capacity of mouse prostate basal and luminal cells during development, tissue-regeneration and transformation. However, evaluating cell-intrinsic and extrinsic regulators of prostate epithelial differentiation capacity using a lineage tracing approach often requires extensive breeding and can be cost-prohibitive. In the prostate organoid assay, basal and luminal cells generate prostatic epithelium ex vivo. Importantly, primary epithelial cells can be isolated from mice of any genetic background or mice treated with any number of small molecules prior to, or after, plating into three-dimensional (3D) culture. Sufficient material for evaluation of differentiation capacity is generated after 7-10 days. Collection of basal-derived and luminal-derived organoids for (1) protein analysis by Western blot and (2) immunohistochemical analysis of intact organoids by whole-mount confocal microscopy enables researchers to evaluate the ex vivo differentiation capacity of prostate epithelial cells. When used in combination, these two approaches provide complementary information about the differentiation capacity of prostate basal and luminal cells in response to genetic or pharmacological manipulation.

Mouse prostate organoids represent a promising context to evaluate mechanisms that regulate differentiation. This paper describes an improved approach to establish prostate organoids, and introduces methods to (1) collect protein lysate from organoids, and (2) fix and stain organoids for whole-mount confocal microscopy.

The prostate epithelium is comprised predominantly of basal and luminal cells. In vivo lineage tracing has been utilized to define the differentiation ...

Testing Targeted Therapies in Cancer using Structural DNA Alteration Analysis and Patient-Derived Xenografts

We present here an integrative approach for testing efficacy of targeted therapies that combines the next generation sequencing technolo-gies, therapeutic target analyses and drug response monitoring using patient derived xenografts (PDX). This strategy was validated using ovarian tumors as an example. The mate-pair next generation sequencing (MPseq) protocol was used to identify structural alterations and followed by analysis of potentially targetable alterations. Human tumors grown in immunocompromised mice were treated with drugs selected based on the genomic analyses. Results demonstrated a good correlation between the predicted and the observed responses in the PDX model. The presented approach can be used to test the efficacy of combination treatments and aid personalized treatment for patients with recurrent cancer, specifically in cases when standard therapy fails and there is a need to use drugs off label.

Here we present a protocol to test the efficacy of targeted therapies selected based on the genomic makeup of a tumor. The protocol describes identification and validation of structural DNA rearrangements, engraftment of patients&#8217; tumors into mice and testing responses to corresponding drugs.

We present here an integrative approach for testing efficacy of targeted therapies that combines the next generation sequencing technolo-gies, therapeutic ...

Evaluating Cell Death Using Cell-Free Supernatant of Probiotics in Three-Dimensional Spheroid Cultures of Colorectal Cancer Cells

This manuscript describes a protocol to evaluate cancer cell deaths in three dimensional (3D) spheroids of multicellular types of cancer cells using supernatants from Lactobacillus fermentum cell culture, considered as probiotics cultures. The use of 3D cultures to test Lactobacillus cell-free supernatant (LCFS) are a better option than testing in 2D monolayers, especially as L. fermentum can produce anti-cancer effects within the gut. L. fermentum supernatant was identified to possess increased anti-proliferative effects against several colorectal cancer (CRC) cells in 3D culture conditions. Interestingly, these effects were strongly related to the culture model, demonstrating the notable ability of L. fermentum to induce cancer cell death. Stable spheroids were generated from diverse CRCs (colorectal cancer cells) using the protocol presented below. This protocol of generating 3D spheroid is time saving and cost effective. This system was developed to easily investigate the anti-cancer effects of LCFS in multiple types of CRC spheroids. As expected, CRC spheroids treated with LCFS strongly induced cell death during the experiment and expressed specific apoptosis molecular markers as analyzed by qRT-PCR, western blotting, and FACS analysis. Therefore, this method is valuable for exploring cell viability and evaluating the efficacy of anti-cancer drugs.

Here methods are presented to understand anti-cancer effects of Lactobacillus cell-free supernatant (LCFS). Colorectal cancer cell lines show cell deaths when treated with LCFS in 3D cultures. The process of generating spheroids can be optimized depending on the scaffold and the analysis methods presented are useful for evaluating the involved signaling pathways.

This manuscript describes a protocol to evaluate cancer cell deaths in three dimensional (3D) spheroids of multicellular types of cancer cells using ...

Immunofluorescence Imaging of DNA Damage and Repair Foci in Human Colon Cancer Cells

The purpose of the manuscript is to provide a step-by-step protocol for performing immunofluorescence microscopy to study the radiation-induced DNA damage response induced by neutron-gamma mixed-beam used in boron neutron capture therapy (BNCT). Specifically, the proposed methodology is applied for the detection of repair proteins activation which can be visualized as foci using antibodies specific to DNA double-strand breaks (DNA-DSBs). DNA repair foci were assessed by immunofluorescence in colon cancer cells (HCT-116) after irradiation with the neutron-mixed beam. DNA-DSBs are the most genotoxic lesions and are repaired in mammalian cells by two major pathways: non-homologous end-joining pathway (NHEJ) and homologous recombination repair (HRR). The frequencies of foci, immunochemically stained, for commonly used markers in radiobiology like &#947;-H2AX, 53BP1 are associated with DNA-DSB number and are considered as efficient and sensitive markers for monitoring the induction and repair of DNA-DSBs. It was established that &#947;-H2AX foci attract repair proteins, leading to a higher concentration of repair factors near a DSB. To monitor DNA damage at the cellular level, immunofluorescence analysis for the presence of DNA-PKcs representative repair protein foci from the NHEJ pathway and Rad52 from the HRR pathway was planned. We have developed and introduced a reliable immunofluorescence staining protocol for the detection of radiation-induced DNA damage response with antibodies specific for repair factors from NHEJ and HRR pathways and observed radiation-induced foci (RIF). The proposed methodology can be used for investigating repair protein that is highly activated in the case of neutron-mixed beam radiation, thereby indicating the dominance of the repair pathway.

Radiation-induced DNA damage response activated by neutron mixed-beam used in boron neutron capture therapy (BNCT) has not been fully established. This protocol provides a step-by-step procedure to detect radiation-induced foci (RIF) of repair proteins by immunofluorescence staining in human colon cancer cell lines after irradiation with the neutron-mixed beam.

The purpose of the manuscript is to provide a step-by-step protocol for performing immunofluorescence microscopy to study the radiation-induced DNA damage ...

In Vitro Evaluation of Oncogenic Transformation in Human Mammary Epithelial Cells

Tumorigenesis is a multi-step process in which cells acquire capabilities&#160;that allow their growth, survival, and dissemination under hostile conditions. Different tests seek to identify and quantify these hallmarks of cancerous cells; however, they often focus on a single aspect of cellular transformation and, in fact, multiple tests are required for their proper characterization. The purpose of this work is to provide researchers with a set of tools to assess cellular transformation in vitro from a broad perspective, thereby making it possible to draw sound conclusions.
A sustained proliferative signaling activation is the major feature of tumoral tissues and can be easily monitored under in vitro conditions by calculating the number of population doublings achieved over time. Besides, the growth of cells in 3D cultures allows their interaction with surrounding cells, resembling what occurs in vivo. This enables the evaluation of cellular aggregation and, together with immunofluorescent labeling of distinctive cellular markers, to obtain information on another relevant feature of tumoral transformation: the loss of proper organization. Another remarkable characteristic of transformed cells is their capacity to grow without attachment to other cells and to the extracellular matrix, which can be evaluated with the anchorage assay.
Detailed experimental procedures to evaluate cell growth rate, to perform immunofluorescent labeling of cell lineage markers in 3D cultures, and to test anchorage-independent cell growth in soft agar are provided. These methodologies are optimized for Breast Primary Epithelial Cells (BPEC) due to its relevance in breast cancer; however, procedures can be applied to other cell types after some adjustments.

This protocol provides experimental in vitro tools to evaluate the transformation of human mammary cells. Detailed steps to follow-up cell proliferation rate, anchorage-independent growth capacity, and distribution of cell lineages in 3D cultures with basement membrane matrix are described.